Accumet XL 20 User Manual
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171Appendix: Determining Isopotenial Points ExperimentallyXL15, 20, 25, 50 and 60 metersThe isopotential point of an electrode system is the point at which electrode potential isunaffected by a change in temperature. The coordinates of this point would be reported as (pXi so ,Eis0 ). For an ideal system, this point would be coincident with the system’s Zero Potential Point(pXiso , E0 ).In practical systems, however, this coincidence rarely occurs, and for some systems, there is notrue Isopotential Point but a general Isopotential area. If a system exhibits an apparentIsopotential Point, or at least an Isopotential area with relatively small spread, Isopotentialcoordinates may be established and possibly used to some advantage. Isopotential correctionmay be used only in conjunction with a One- or Two- Point Standardization; it is required onlywhen both accuracy over a significant temperature range as well as operation with an asymmetricelectrode system are contemplated. However, Isopotential correction is never necessary if allmeasurements will be performed on samples which are at similar temperatures. Mostconventional pH electrode systems are designed and manufactured to be highly symmetricalcells. Consequently, for most pH work, unless ultimate accuracy over broad temperature rangesis required, Isopotential may be ignored. Specifically, this is accomplished by setting theIsopotential at its reset value of zero millivolts.The following is an example for determining an Isopotential Point:Data points are first established for three different standards at three different temperatures. Thedata points are then plotted and will produce three isotherms which should resemble the plot ineither figure 1 or figure 2. In figure 2, the Isopotential Point is well defined as the intersection pointof the three Isotherms (pX so , E iso ). The Isopotential Point is not so well defined in figure 1, but avalue may be interpolated with some accepted error. |
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